Marizomib is a proteasome inhibitor capable of inhibiting all three domains of the proteasome (i.e., the chymotrypsin-like (CT-L); caspase-like (C-L) and trypsin-like (T-L) domains). Accordingly, marizomib can be useful for treating diseases such as cancer. Thus, there is a need for pure, stable morphic forms of marizomib that can be used for administration to subjects in need thereof. The present disclosure teaches stable and pure morphic forms of marizomib.